The Federal Aviation Administration (FAA) requires all passenger carrying aircraft over 30 seats be equipped with so-called “Mode S” transponders. Mode S transponders are capable of transmitting a number (e.g., 25) of formats of coded data. This coded data includes such information as a unique 24-bit binary address for each aircraft.
The aircraft registration number may be derived from this 24-bit binary address. The coded Mode-S data also includes such information as aircraft altitude and is transmitted continuously throughout a flight at a minimum rate of 1 Hz (i.e., once per second). Coded Mode-S data may be collected passively without any connection to air traffic control equipment.
The FAA has endorsed the Aircraft Communications Addressing and Reporting System (ACARS) system, which uses various data link technologies including the VHF communication band, HF and SATCOM along with a ground station network to allow aircraft to transmit and receive messages of coded data. Many domestic and international carriers have equipped their aircraft with ACARS equipment.
ACARS equipment is capable of transmitting a number of types of coded data. ACARS currently uses frequency shift keying (FSK) as a modulation scheme, however, other modulation schemes including minimum shift keying (MSK) and time division multiple access (TDMA) are being evaluated for future improvement of ACARS. ACARS data includes such information as the aircraft registration number and airline flight identification number (flight number).
ACARS transmissions from a single aircraft may be sent at varying intervals from as little as no transmissions in a single flight to several transmissions per minute. ACARS transmissions may be collected passively without any connection to air traffic control equipment.
None of the currently used or planned Mode S downlink formats provides for the transmission of flight identification data. There are a number of methods including Automatic Dependent Surveillance—Broadcast (ADS-B) and multilateration which allow for the precise determination of aircraft location through the Mode S downlink formats on a frequent basis. ACARS transmissions, while capable of encoding aircraft position and altitude, are not typically used for position determination as the frequency of ACARS transmissions is too infrequent to allow one to accurately and timely determine the exact position of an aircraft.